Wednesday, 25 August 2010

World's Trees Fail to Soak Up Carbon Pollution

Planting lots of trees isn't enough to save us from global warming. Higher temperatures and lower rainfall over the last few years have meant forests are soaking up less rather than more carbon dioxide.

This is the conclusion of a study just published in the journal Science. In the 20 years leading up to 2000, plant growth increased by 6% as warming extended the growth season for most regions. But then, a small but significant decline by 1%. A drought in the Amazon basin in 2005 is probably the major cause of this decline but the trend is true across the world.

Ben Cubby in today's Sydney Morning Herald quotes Josep Canadell, from CSIRO, who says that Australia is also a major contributor to the global decline. He says "There has been a measurable decline in the leaf area of plants this decade, though we don't have all the data from Australia yet."

NASA satellite data was used to measure forest cover around the world between 2000 and 2009. In some areas higher temperatures led to increased plant growth but these gains were matched by the drying out of rainforest areas. Overall there was a decline in carbon dioxide being taken up by forest.

According to Cubby, scientists in Australia are worried about a "vicious cycle" where "trees absorb less carbon because the world is warmer and drier, while the rising carbon levels in the atmosphere continue to trap heart".

Other studies support this conclusion. A report by Matthew Berger on the SolveClimate site mentions a PLoS ONE (you can serarch for this one!) study that showed increasing carbon in the atmosperhe will eventually lead to decreased storage of carbon by plants. This is largely because plants suffer water stress before they can take advantage of the increased carbon dioxide.

Andy Pitman from University of New South Wales says part of the solution is to protect old-growth forests and replant areas where forests have been cleared. However he says the root cause is still human activity emitting carbon dioxide.

The Climate Progress report on this study concludes with the following sobering remarks: "This new study is the latest in a series of truly alarming scientific studies published just this year that suggest we may be closer to dangerous carbon-cycle tipping points and irreversible thresholds than anyone realised...Failure to take strong and immediate action to reduce greenhouse gas emmissions is increasingly looking like societal suicide."

Image: Planting a tree in Sydney's Royal Botanic Gardens with Alan Leishman